The temperate bacteriophage Mu is characterized by the highly promiscuous integration of its DNA into the genome of its host bacterium Escherichia coli. We are analyzing the life cycle of Mu by a combination of genetic, biochemical, and electron microscopic techniques. We wish to unravel the mechanism by which Mu is integrated into the host chromosome and to isolate and purify proteins which bring about this random nonhomologous integration. Our immediate plan is to focus on Mu DNA--its structure, its intracellular forms, its mode of replication--and on the genetics of Mu excision--functions involved in excision, products of excision. Exploration of Mu DNA in vivo and in vitro will lead us to functions which govern its intramolecular and intermolecular reactions. The excision genetics will give us a better insight into the interaction of Mu and host genomes and will allow us to compare the Mu phenomenon with other translocation elements. We hope that studies on Mu will not only contribute to the improvement of techniques for precisely manipulating various genomes but also throw light on the process of integration of viruses in eukaryotic cells. We also propose to use mutants of E. coli, sensitive to bile salts, as hosts for DNA recombinant molecules. An EK3 E. coli host-vector system, conforming to the recommended National Institutes of Health Guidelines, will be developed for cloning of DNA molecules.